1. Field of the Invention
The present invention pertains to an ultra-high frequency oscillator with low noise in the vicinity of the carrier frequency. This oscillator is of the doubling type, i.e. the frequency delivered to the load is double the oscillation frequency of the transistors in the so-called push-push mode of assembly. A decrease in the noise close to the central frequency is obtained by means of an optimum low-frequency impedance placed between the gates of the two transistors, this circuit either placing the non-correlated noise sources in series or preventing the low-frequency noise sources of the transistors from being grounded.
It is important for both present-day and future ultra-high frequency oscillators, which work at several tens of gigahertz, that the frequency which they deliver to a load is as pure as possible, i.e. that there is no spurious oscillation within a band of frequencies close to the central frequency known as the carrier: for example, in a band of ten kHz around 20 GHz.
2. Description of the Prior Art
There are various means in the prior art to obtain this result. They vary according to the pattern of the oscillator considered. For example, in a feedback oscillator, the limiting and amplifying functions can be separated, but this type of oscillator requires transistors which oscillate at 10 GHz if the carrier is at 100 GHz. Transistors of this type are very small in size and lack power.
3. Summary of the Invention
The so-called push-push assembly used in the invention has the advantage of requiring only two transistors which oscillate at 50 GHz if a 100 GHz carrier is sought. In this assembly, the two transistors are mounted in parallel and their gates are linked to two microstrips, parallel to each other, between which a ceramic dielectric resonator is positioned. The microstrips are grounded through a resistor, conventionally of 50 ohms. If each transistor is considered separately, the resonating circuit comprising the microstrip and the dielectric resonator is well known, but each of the two resonating circuits does not cancel the low-frequency noise at the transistors.
According to the invention, the sources of non-correlated low-frequency noises of field-effect transistors are:
Either loaded with an infinite impedance by means of a high-value capacitance placed between the end of the gate microstrip and the grounded resistor. This capacitance prevents the grounding of the low-frequency noise sources and hence reduces the medium-frequency noise at the second harmonic,
Or placed in series by means of a choke which joins the two transistor gate microstrips to each other, this choke being soldered at points located between the dielectric resonator and the end of the microstrip joined to a resistor, such that the low-frequency noise around the second harmonic in the load is reduced and shifted to a distance away from the carrier. The sources of the low-frequency noises of the transistors can be placed in series because, for each microstrip, a capacitance is interposed between the end of the microstrip and the grounded resistor: the value of these capacitances is high enough for the capacitance to be equivalent to a short circuit at ultra-high frequencies and act as an open circuit at low frequencies, and for the preceding choke to join the two gates as regards low-frequencies.
More precisely, the invention pertains to a frequency-doubling oscillator, working at ultra-high frequencies, comprising, in a so-called push-push assembly, two field-effect transistors working in parallel, with their source and drain electrodes connected to impedances, the working load being connected to a point common to the impedances of the output electrodes of both transistors, the gates of each transistor being connected, by an impedance, to a first end of a microstrip line, the second end of which is grounded by a resistor, a dielectric resonator being placed between the two microstrip lines at a point such that oscillation is obtained. In the disclosed oscillator, in order to reduce the low-frequency noise in the load near the main frequency, known as the carrier frequency, the noise sources at the transistors are insulated from the ground at direct current and loaded with a low-frequency impedance of infinite value, consisting of two high-value capacitances, each of these two capacitances being interposed between the second end of a microstrip line and its grounding resistor.